The present invention relates to a crosslinkable vinylidene fluoride polymer composition, to a process for crosslinking the composition and to crosslinked shaped articles.
By virtue of their greater flexibility, by comparison with the poly(vinylidene fluoride) homo-polymer, vinylidene fluoride copolymers and their mixtures with poly(vinylidene fluoride) have many applications in fields such as the sheathing of electrical cables or the preparation of flexible fuel pipes. In these applications, it is highly desirable to be able to combine their inherent properties, such as flexibility and high resistance to fire and to corrosion, with improved mechanical and thermomechanical (after heat aging) properties.
It is known that the crosslinking of polymers and in particular of thermoplastic polymers generally causes an improvement in the mechanical properties and in the chemical resistance, as well as an improvement in their impermeability. However, it is most often accompanied by a relatively great stiffening.
A. Vokal and M. Pallanova (cf. 7th Symp. on Radiation Chemistry, 1990, pp. 311-315) have studied the crosslinking of homopolymers and of copolymers of vinylidene fluoride (VF2) with hexafluoropropylene (HFP) in the presence of crosslinking promoters under the effect of ionizing radiation at doses of 100 to 400 kGray (10 to 40 Mrad). This study shows that VF2-HFP copolymers are easier to crosslink than the homopolymer, that is to say that for identical doses of irradiation, the copolymers exhibit markedly higher contents of gel (levels of insolubles in a solvent for the non-crosslinked copolymer) than the homopolymer. Nevertheless, the crosslinking of VF2-HFP copolymers takes place to the detriment of the breaking stress and of the flexibility of the shaped items.
The present invention is targeted at providing a crosslinkable composition based on vinylidene fluoride polymers which does not exhibit this disadvantage.
To this end, the invention relates to a vinylidene fluoride polymer composition which can be crosslinked under the effect of an ionizing radiation, characterized in that the vinylidene fluoride polymer comprises a thermoplastic copolymer of vinylidene fluoride and of chlorotrifluoroethylene and in that the composition contains an effective amount of crosslinking promoter.
The vinylidene fluoride polymer present in the crosslinkable compositions according to the invention can be composed essentially of thermoplastic copolymers of VF2 and of CTFE. It can also be composed of a mixture of such copolymers and of vinylidene fluoride homopolymer. In this case, the copolymer of vinylidene fluoride and of chlorotrifluoroethylene and the vinylidene fluoride homopolymer are most often present in a ratio by weight of 75/25 to 25/75.
Thermoplastic copolymer of vinylidene fluoride and of chlorotrifluoroethylene is understood to denote, for the purposes of the present invention, vinylidene fluoride copolymers which contain up to approximately 25% by weight of chlorotrifluoroethylene. Advantageously, they contain at least 10% by weight thereof. Preferably, they contain from 12 to 22% by weight thereof. These preferred copolymers exhibit melting temperatures from approximately 170 to approximately 165xc2x0 C.
The nature of the crosslinking promoter is not critical. The latter can therefore be chosen from conventional promoters of crosslinking under the effect of an ionizing radiation. Mention may be made, as non-limiting examples of promoters which can be used, of triallyl cyanurate and isocyanurate, allyl esters of poly-carboxylic acids, such as diallyl phthalate and tetra-allyl pyromellitate, bismaleimides, such as N,Nxe2x80x2-ethylenebismaleimide, or multiacrylates, such as dipenta-erythritol hexamethacrylate. Preference is nevertheless given to triallyl cyanurate and to triallyl isocyanurate and more particularly still to the latter, which has a beneficial effect on the lubrication of the crosslinkable compositions according to the invention.
The effective amount of crosslinking promoter is understood to denote an amount which is sufficient to promote crosslinking under the effect of ionizing radiation. Although this amount is not particularly critical, use is generally made of at least 0.5 part and most often of at least one part by weight of crosslinking promoter per 100 parts by weight in total of vinylidene fluoride polymer. Generally, a content of 5 parts by weight is not exceeded. Advantageously, the crosslinking promoter is present in an amount of 2 to 4 parts per 100 parts by weight in total of vinylidene fluoride polymer.
Particularly preferred compositions according to the invention contain a copolymer of vinylidene fluoride and of chlorotrifluoroethylene containing from 12 to 22% by weight of chlorotrifluoroethylene, optionally as a mixture with poly(vinylidene fluoride) homopolymer in the ratio by weight 70/30 to 30/70, and from 2 to 4 parts of triallyl isocyanurate (crosslinking promoter) per 100 parts by weight in total of vinylidene fluoride polymer.
It is understood that, in addition to the essential constituents which constitute the vinylidene fluoride copolymer, optionally vinylidene fluoride homopolymer, and the crosslinking promoter, the crosslinkable compositions according to the invention can comprise all the usual processing ingredients of compositions based on fluorinated polymers, such as, for example, lubricants, as well as various additives, such as fillers, pigments and/or dyes, flame-retardant agents, smoke-suppressants, and the like.
The crosslinkable compositions according to the invention can be manufactured in any known conventional way for mixing polymers and their various processing ingredients. They can, for example, be manufactured by mixing, in a single or in a number of operations, all the ingredients in a low-temperature fast mixer in order to produce non-pregelled compositions existing in the powder form, generally known as premixes. They can also be manufactured by mixing, in a single or in a number of operations, all the ingredients in an extruder in which the mixture is brought to a temperature at least equal to the melting temperature of the vinylidene fluoride polymer in order to produce pregelled mixes, generally known as compound mixtures. The crosslinkable compositions according to the invention are advantageously provided in the compound form and in particular in the form of granules.
The compositions according to the invention can be processed by all the conventional techniques for the conversion of plastics in the molten state, such as extrusion and injection. They are particularly suitable for being processed by extrusion.
The crosslinkable compositions according to the invention are readily crosslinked in air under the effect of relatively low doses of ionizing radiation.
The invention also relates to a process for the crosslinking of the compositions according to the invention. According to this process, the composition is irradiated in air by means of an ionizing radiation at a dose which does not exceed 100 kGray. This dose most often amounts to at least 10 kGray. Excellent results are obtained with irradiation doses of 50 to 90 kGray.
The nature of the ionizing radiation is not critical in carrying out the crosslinking process according to the invention. The ionizing radiation can be composed, for example, of xcex2-rays (accelerated electrons) or alternatively of xcex-rays (emitted by a cobalt source). Nevertheless, due to their greater accessibility, preference is given to xcex2-rays.
The compositions crosslinked according to the process of the invention exhibit high levels of crosslinking. Crosslinking is reflected in practice by the level of gel. The latter is evaluated by immersion of an irradiated polymer sample in a boiling solvent which dissolves the non-crosslinked polymer fraction. The content of insoluble materials, expressed in percent, corresponds to the level of gel representative of the level of crosslinking. Generally, the level of gel of the compositions crosslinked according to the process of the invention (evaluated by immersion for thirty minutes in boiling dimethylformamide) amounts to more than 50% and, most often, to more than 60%.
An advantage of the compositions crosslinked according to the invention is the fact that they exhibit an improved tensile breaking stress in comparison with an identical non-crosslinked composition, including after severe heat aging, for example at temperatures exceeding 200xc2x0 C. It is true that the crosslinked compositions exhibit a reduced flexibility in comparison with an identical non-crosslinked composition. Nevertheless, the flexibility of the crosslinked compositions remains superior to that of a non-crosslinked vinylidene fluoride homopolymer and, moreover, largely sufficient to satisfy the standards laid down in certain fields of application, such as that of cable manufacture.
In addition, it has been found that, when the crosslinked composition is subjected to heat aging, the flexibility is significantly improved to the point of being able to achieve values equivalent to, if not greater than, those of non-crosslinked compositions.
According to a specific embodiment of the crosslinking process according to the invention, the composition is subjected after irradiation to a heat treatment. This heat treatment can be carried out at very variable temperatures and for equally variable time periods. To give an idea, the temperature will generally amount to at least 80xc2x0 C. and most often to at least 100xc2x0 C. The latter will not generally exceed 215xc2x0 C. and most often it will not exceed 200xc2x0 C. The duration of the heat treatment can range from a few hours to a few days. The optimum conditions for the heat treatment, temperature and duration, will advantageously be evaluated experimentally in each specific case.
The crosslinkable compositions according to the invention are capable of being shaped into highly diverse articles which, after crosslinking and heat aging, combine excellent mechanical properties with excellent flexibility.
Due to these advantageous properties, the crosslinkable compositions according to the invention are particularly suitable for the sheathing of electrical cables, such as communication cables, and for the extrusion of pipes intended for the transportation of fuel in automobiles (fuel lines) or alternatively of multilayer structures intended for the transportation of oil, in particular in marine environments (offshore).
The invention also relates to the crosslinkable shaped articles resulting from the conversion in the molten state of the crosslinkable composition according to the invention. It also relates to the crosslinked shaped articles resulting from the crosslinking by means of the process of the invention of the crosslinkable shaped articles.
These crosslinkable and crosslinked shaped articles are composed, preferably, of sheaths for electrical cables and of pipes for oil products.